After I fuly charged my iPod Mini battery, i plugged it into my PC to upload somesongs. I left it on the dock for about an hour. I took it off and the battery was almost fully drained. This happend before. Why does it drain the battery? when its plugged in and says "Do Not Disconnect" it shows the batery filling up in the corner with a jolt in from. I thought that meant it was chanrging. Why would it die that fast?